The use of mineral fillers or pigments to enhance the properties of products such as paints, papers, rubbers, polymers, plastics or other similar materials is well known. One particular application involves the utilization of such products as flatting agents in paint and coating formulations for reducing the sheen and gloss properties of the same. Many of the mineral fillers and pigments used as flatting agents also find utility in plastic film applications as anti-block agents.
One type of flatting agent for paint and/or other coatings is diatomaceous silica. Diafil(copyright) 525 flatting agent (available from C. R. Minerals) is an example of a diatomaceous silica flatting agent which exhibits good flatting performance in paints. Calcination of a diatomaceous silica is often necessary to achieve the higher pigment brightness and whiteness properties typically desired by paint formulators. Calcined diatomaceous silica products also find utility as anti-block agents for plastic film such as polyethylene (PE) film compositions. These anti-block agents are typically used at low addition levels so as to preserve film clarity while still preventing the PE film from sticking to itself. However, the calcination process leads to the formation of large amounts of crystalline silica in the calcined diatomaceous silica product. Such amounts of crystalline silica are undesirable from a worker safety viewpoint. Specifically, the inhalation of crystalline silica is considered to be a serious health hazard.
Another type of flatting and anti-block agent is disclosed in U.S. Pat. No. 5,167,707 to Freeman et al. (xe2x80x9cFreemanxe2x80x9d), the contents of which are herein incorporated by reference. In this patent, a coarse particle, structured pigment having a sodium alumino-silicate composition (SAMS), prepared from the hydrothermal reaction of delaminated kaolin clays with select sodium silicate reagents, is disclosed. The SAMS pigment described in Freeman was developed as an improvement to existing flatting agents, including the diatomaceous silica types noted above. The patent identifies certain SAMS pigment properties that are preferred for flatting performance when used in paints and for anti-block performance in PE films. More particularly, the SAMS pigment has an oil absorption of from about 90 to 110 g/100 g; an average particle diameter from 12.8-14.8 microns; a specific surface area from 0.68-0.78 m2/g; a BET surface area from 3.0-9.0 m2/g; a total pore volume as determined by mercury intrusion of 1.2-1.8 ml/g; a pore structure with a pore diameter centered at 2.4 microns as determined by log differential intrusion analysis, and a general chemical formula of (0.6-1.6) Na2O:Al2O3:(4.2-7.5)SiO2:(1.0-5.0)H2O.
The flatting and anti-block agents noted above are not without their disadvantages. For example, the calcined diatomaceous silica products contain crystalline silica at levels which the federal government has identified as carcinogenic. Therefore, specific labeling must be employed when this agent is used in paints. Consequently, there is a considerable desire in the paint and coating industry to find an alternative to replace diatomaceous silica, particularly calcined diatomaceous silica, as a flatting agent.
The health risks associated with the use of crystalline silica in paints is avoided by using the SAMS pigment described in Freeman. However, since this SAMS pigment is derived from a kaolin clay feedstock, the whiteness and brightness values thereof can still be inferior to diatomaceous silica products, particularly those that are calcined. Kaolin clays are typically yellowish white in color owing to the presence of iron oxides and other natural mineral impurities and this color carries over to the final SAMS pigment despite the hydrothermal processing conditions employed in its making.
In view of the deficiencies in the prior art flatting agents noted above, a need exists for alternative flatting agents that do not contain crystalline silica and that do not have a negative impact on pigment brightness, whiteness or other properties. Anti-block agents that do not contain crystalline silica are also desired.
The present invention solves these needs through the development of a highly-structured alumina monohydrate (boehmite) pigment. The structured pigment is made by reacting alumina trihydrate feedstock under hydrothermal conditions at elevated temperatures and pressures in the presence of an alkaline earth metal base to form aggregated plates of the alumina monohydrate.
Alumina trihydrate (i.e., ATH or gibbsite) is a well known pigment in the paper and paint industries. It is frequently used as a white extender pigment in the paper industry. Alumina trihydrate is also used as a raw material for the production of other aluminum compounds such as the preparation of activated aluminas, which are commonly used as absorbents and catalysts. Alumina trihydrate can also be used as a filler in plastics and rubbers due to its fire-retardant and smoke-suppressant properties.
The conversion of gibbsite (alumina trihydrate) into boehmite (alumina monohydrate) under hydrothermal conditions is disclosed in U.S. Pat. No. 5,306,680 to Fukuda. Boehmite is one mineralogical form of alumina monohydrate (AlOOH), that is derived from gibbsite. In the present invention, a fine, flaky boehmite particle is formed by subjecting submicron alumina trihydrate to hydrothermal treatment in water or an alkali solution at elevated temperatures and pressures. The fine, flaky boehmite particles produced by this treatment are useful as a starting material for the preparation of fine, flaky alumina particles or as a filler for rubbers and plastics and as a coating pigment material for paper making.
Alumina trihydrate has also been substituted for Celite(copyright), a calcined diatomaceous silica flatting agent in paint applications available from the Johns-Manville Corporation of Denver, Colo. The publication entitled xe2x80x9cFormulation of Flat Latex Coatings With Aluminum Trihydroxide As An Extender Pigment for Titanium Oxide,xe2x80x9d by Stoffer, et al., American Chemical Society, Division of PMSE, Papers 1993, 569, pp. 384-385, demonstrates that alumina trihydrate can be used successfully as an extender pigment for titanium oxide in flat latex paints.
While the prior art recognizes that boehmite can be made via hydrothermal treatment of alumina trihydrate and that alumina trihydrate can be used in paint applications, the prior art does not teach nor suggest making a highly structured pigment of alumina monohydrate having the properties of the instant invention nor a hydrothermal treatment for making the same.
The invention comprises a structured boehmite pigment comprising aggregated platelets of alumina monohydrate having physical properties making it suitable for applications in paints, papers, rubbers, polymers and the like. Preferably, the alumina monohydrate product is in the boehmite crystal form as derived from the hydrothermal conversion of gibbsite.
The structured boehmite pigment is made up of platelets of alumina monohydrate which are arranged in a stacked or aggregated configuration. The platelets may be either edge-to-edge, face-to-face, edge-to-face or combinations thereof. The platelets making up the structured pigment are in the micron range, generally ranging in size from about 0.5 to 3.0 microns. The platelet size and resultant structured aggregate particle size can vary depending on the processing conditions as described below.
The physical properties of the structured boehmite pigment make it ideal for pigment applications, particularly as a flatting agent for paint and as an anti-block agent for film. The structured pigment has a total pore volume of at least 0.8 ml/g and, more preferably, of between about 0.8 to 2.5 ml/g. The structured pigment is characterized as having a low to medium structure as defined in Freeman, see column 10, lines 1-15, wherein structure is measured in terms of a pigment""s total pore volume as determined by Hg Intrusion Porosimetry.
The aggregate median particle size, as measured by Malvern (LLS) median particle size measurement, is at least about 8 microns and can go as high as about 30 microns. A more preferred median particle size range is between about 10 to 15 microns. In conjunction with the desired median particle size, screen residue residuals for the product should be less than 1% greater than 325 mesh, preferably less than 0.1%. Screen residue values at 325 mesh have a direct bearing on the Hegman Grind values determined in a paint formulation. Such values indicate the smoothness of a paint film.
The structured boehmite pigment has a low differential pore volume (DPV) since low opacity, particularly when the pigment is used as a flatting agent in black or colored enamels, is desirable. A desirable differential pore volume for the inventive structured pigment is a maximum of about 0.3 ml/g, and is preferably less than 0.2 ml/g. The DPV value is the amount of pigment pore volume, as measured in ml/g, that occurs between the pore diameter size range of 0.1-0.4 microns. This is the range of pore sizes that provide the most efficient scattering of visible light. A low pigment DPV value therefore typically translates to low opacifying properties.
Preferred oil absorption values for the structured boehmite pigment of the instant invention vary from 70 to 135 ml/100 g and, more preferably, between 90 and 105 ml/100 g.
The structured boehmite pigment has a BET surface area range of about 3 to 20 m2/g and, more preferably, about 5 to 10 m2/g.
The structured boehmite pigment is made by first providing a feedstock of alumina trihydrate or aluminum hydroxide particles. The feedstock is hydrothermally treated in the presence of at least water and an alkaline earth metal base under conditions of elevated temperatures and pressures to form the aggregated platelets of alumina monohydrate. Although any alkaline earth metal base can be used, preferred bases include calcium hydroxide, barium hydroxide, strontium hydroxide and magnesium hydroxide or their oxides (CaO, BaO, SrO and MgO).
Although the feedstock can be any type of alumina trihydrate, a preferred feedstock is a dry ground alumina trihydrate with a medium particle size range between about 9 and 10 microns. Other feedstock types and sizes can be utilized as would be within the skill of the art.
It is preferred that the reaction process be conducted with the ATH feedstock as an aqueous slurry, preferably at a reaction solids percent of 8-35% and more preferably at about 27-32%. The reaction temperature can range between about 150-250xc2x0 C., the temperature varying with the pressure as it is known from the relationship of temperature and pressure from standard steam tables. The time of the hydrothermal treatment will vary with the temperature. The higher the reaction temperature, the less time needed to convert the alumina trihydrate (gibbsite) to the desired alumina monohydrate (boehmite) product. Typically, the hydrothermal conversion from the trihydrate to the monohydrate takes about 20-90 minutes.
The structured boehmite pigment TAPPI brightness is at least 90, such as in the range of about 94 to about 98 and is typically in the range of 94-97.
The structured boehmite pigment can be used to enhance the properties of any material, but is particularly suited for use as a flatting agent in paints and as an anti-block agent for plastic film. The structured pigment of the invention also overcomes the drawback of calcined diatomaceous silica products by eliminating the crystalline silica without losing brightness or whiteness. The pigment is superior to the kaolin clay-based SAMS pigments by reason of its higher brightness. The inventive pigment is advantageous in that it can be employed as a drop-in substitute for a calcined diatomaceous silica type flatting agent (such as Celite(copyright) 281 flatting agent) or as a drop-in substitute for a calcined diatomaceous silica type anti-block agent (such as Superfloss(copyright) anti-block agent), without the need to reformulate the paint or film compositions.